1. Technical Field
The present invention relates to a method of manufacturing an optical fiber base material being capable of constantly manufacturing an optical fiber base material of high quality by so-called VAD, and an apparatus of the same.
2. Related Art
VAD is well-known as a method of manufacturing base materials for optical fibers. This method employs the following apparatus, for example.
In this apparatus, glass particles produced with a core deposition burner and a cladding deposition burner disposed in a reaction chamber; and the glass particles are deposited onto a tip of a starter mounted on a shaft which rotatably lifts up, so that a porous glass base material for optical fiber composed of a core layer and a cladding layer is manufactured. The core layer may be SiO2 with which GeO2 is doped, and the cladding layer may be substantially pure SiO2.
The porous glass base material 1 manufactured as described above is dehydrated and sintered in a heating furnace. The heating furnace has a furnace tube 2 which can be sealed, an electric furnace 3 which heats a part of or the whole of the furnace tube 2, a gas introducing port 4 which introduces any gas into the furnace tube and a gas discharging port 5 which discharges the exhaust gas as shown in FIG. 1, for example. FIGS. 1A to 1C progressively show vitrifying the porous glass base material. Here, reference numeral 6 indicates a shaft which supports the porous glass base material 1.
Dehydrating is performed by heating the base material at approximately 1,100 degrees Celsius in dehydrating gas composed of such as chlorine, oxygen and helium. Meanwhile, vitrifying is performed by heating the base material at approximately 1,500 degrees Celsius in an atmosphere containing such as helium.
For the furnace tube forming a part of the heating furnace, conventionally a silica tube made of natural quartz has been employed as described in Japanese Patent Application Publication No. 2004-002109. For example, the silica tube may be an electric-furnace-melted natural quartz glass tube such as HERALUX-E (trade name), available from Shin-Etsu Quartz Products Co., Ltd., which is made by pulverizing natural quartz and melting in an electric furnace (herein after referred to as a natural quartz tube).
The optical fiber base material manufactured as above may be formed as a complete optical fiber base material by adding a cladding to the periphery thereof.
An optical fiber is obtained by drawing the optical fiber base material manufactured as above, and is provided for optical signal transmission. For example, light having a wavelength of 1,310 nm and 1,550 nm is modulated and transmitted through a single-mode fiber.
Usually, the transmission loss of the optical fiber at the wavelength of 1,310 nm is about 0.32 to 0.34 dB/km, however, it could infrequently become higher than usual, such as about 0.34 to 0.36 dB/km. In most cases, the transmission loss of the optical fiber at the wavelength of 1,550 nm is not very higher than a normal value. Moreover, when transmission losses for a wide wavelength range, such as 900 nm to 1,600 nm are examined, the shorter the wavelength is, the larger the transmission loss is. Conventionally, such transmission loss has been acceptable, however, the market strictly requests for an optical characteristic in recent years, therefore, such transmission loss has come under a problem.